Presses for compacting material into a traveling web or board, such as wood chipboard, basically have upper and lower continuously traveling metal belt spans which are vertically interspaced so that the starting material can be introduced and carried between the spans. Means respectively above and below the spans directly conductively heat the spans while pressing them towards each other so that the starting and gradually compacting material carried between the spans can be ultimately heated and compacted to produce the desired product such as chipboard. The means used to press and heat the spans can be relatively complicated so that friction between the means and the traveling belt span is not excessive. Therefore, if the means becomes contaminated by the material used, it can possibly be rendered inoperative.
Therefore, it is customary to make the belt spans wider than the transverse area encompassed by the heating and pressing means so as to shield the latter from possible contamination. In addition, the wider belt spans permit some transverse wandering of the spans.
The heating-pressing means conducts heat directly into the major transverse portion of the metal belt spans, but the overhanging or transversely extending side portions are heated only by conduction through the relatively thin metal belt and they are exposed to the ambient atmosphere. With the traveling belt spans made as usual, a condition results wherein the side edge portions of the belts must operate at a substantially lower temperature than the balance of the belt material between the side portions. Consequently, when going from ambient temperature to press-operated temperature, the belt spans thermally expand longitudinally, excepting for their cooler side edge portions, this throwing these side edge portions into tension. The tension stress and other resulting stresses causes deformation of the belt spans. Such deformation is generaly undesirable, but at the feeding end of the belt spans can result in uneven feeding of compacted material which is to travel between the belt spans while being heated and compressed.
The above indicated problem has been treated by the West German Patent DE-OS 22 43 465. However, this concerns heating the side edge portions of the belts under the control of instrumentation which is very expensive.